Elsevier

Electrochimica Acta

Volume 54, Issue 18, 15 July 2009, Pages 4389-4396
Electrochimica Acta

Effects of phosphate species on localised corrosion of steel in NaHCO3 + NaCl electrolytes

https://doi.org/10.1016/j.electacta.2009.03.014Get rights and content

Abstract

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO3 + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential Eb with the phosphate concentration was investigated by linear voltammetry. Eb increased from −15 ± 5 mV/SCE for [HPO42−] = 0 to 180 ± 40 mV/SCE for [HPO42−] = 0.02 mol L−1. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na2HPO4 solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO42−] = 0.004 mol L−1, siderite FeCO3 was detected first. It was oxidised later into carbonated green rust GR(CO32−) by dissolved O2. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO32−) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO42−] = 0.02 mol L−1, siderite was accompanied by an amorphous precursor of vivianite, Fe2(PO4)3·8H2O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.

Introduction

Orthophosphates are generally considered as effective corrosion inhibitors for iron and carbon steels in aqueous environments and in particular in bicarbonate media [1]. For instance, they are present in drinking water distribution systems [2], are envisioned as inhibitors for steel reinforcement in concrete [3], [4] and are incorporated as zinc phosphate to anticorrosive paints (e.g. [5], [6], [7]). They are classified as non-oxidising anodic inhibitors, and are only efficient in the presence of oxygen. Their advantage is a relatively low cost and a relatively low toxicity in comparison with alternative inhibitors such as nitrite, formerly used as a corrosion inhibitor in concrete, or chromate, still used in paints.

The nature of the film forming on iron in phosphate solutions (typically a 0.1N sodium orthophosphate solution) depends on various parameters such as potential applied to the electrode, the presence of oxygen but it is more particularly strongly related to pH (e.g. [8], [9]). For instance, the film formed on iron by anodic polarisation in slightly alkaline media (pH 8–9) is composed of an inner part of iron oxide(s) and an outer part of iron phosphate(s) [10]. An increase of pH favours the iron ox(yhydrox)ides and for instance the passive film at pH 12 mainly consists of an iron oxide [9]. Such passive oxide films are characterised by a spinel structure similar to that of magnetite and maghemite, as observed in various media [11], [12], [13], [14]. In contrast, acidic and neutral media favour the formation of iron phosphates. At pH 7, steel is thus covered with a layer of iron phosphate(s) [15]. As a matter of fact, the principle of phosphatation of steel involves the reaction of the metal with a solution of phosphoric acid.

The efficiency and the mechanisms of the inhibiting action of phosphates, and in particular with respect to localised corrosion processes, are not still completely clarified. It is still thought that the inhibiting effect is mainly due to the competitive adsorption between phosphate and chloride ions on the passive layer that retards the destructive action of chloride [16]. It was also observed that phosphate could favour the repassivation of pits, but mainly in stirred electrolytes [17]. This repassivation was attributed in the case of stainless steels to the buffering effects of phosphate [18]. It is then assumed that pH remains sufficiently high inside the pits to allow the re-formation of the protective iron oxide based passive film. In contrast, the repassivation of carbon steel was attributed to the formation of protective phosphate-containing iron compounds inside the pits [19], [20].

In the case of pitting processes of stainless steels in environments containing chloride ions, the formation of a non-porous salt film is a condition necessary for the growth of the pit. It forms a barrier that lower significantly ion transport and maintains the high chloride concentration and acidity required for active dissolution of iron inside the pits [21]. Recent characterisation by synchrotron X-ray diffraction revealed that the salt film was predominantly made of FeCl2·4H2O [22]. Previous X-ray fluorescence studies already indicated that the salt layer in (Cr, Ni, and Fe) and (Cr, Ni, Mo, and Fe) stainless steels was rich in iron [23], [24]. In the case of carbon steel in bicarbonate media, siderite FeCO3, because of its very low solubility, precipitates inside the pits as soon as the anodic dissolution of iron begins [25]. The reaction isFe2+ + HCO3  FeCO3 + H+So, the precipitation of FeCO3 induces itself the acidification of the electrolyte inside the pits, and the subsequent increase of the chloride concentration due to the fact that Cl ions do not precipitate with Fe2+ and tend to accumulate to counterbalance the H+ ions produced.

With this study, we intended to obtain new information on the mechanisms of the localised corrosion processes and/or the mechanisms of repassivation of carbon steel in bicarbonate media in the presence of phosphate. This was achieved by coupling spectroscopic and electrochemical microprobes, as previously done to clarify the mechanisms involved in the inhibition of the corrosion of steel by NO2 ions [26]. Micro-Raman spectroscopy and scanning vibrating electrode technique (SVET) were then used to monitor in situ the pitting processes of carbon steel in NaHCO3 (0.1 mol L−1) + NaCl (0.02 mol L−1) + Na2HPO4 (0.004 or 0.02 mol L−1) solutions. The carbonate and chloride concentrations were taken equal to those used in previous studies [25], [26], in order to facilitate comparison. A preliminary study of the behaviour of steel was performed by linear voltammetry.

Section snippets

Electrochemistry

Electrochemical experiments were carried out in a classical three-electrode glass cell. The working electrodes were carbon steel disks with 2 cm2 area. The steel approximate composition (in weight%) was: 98.2% Fe, 0.122% C, 0.206% Si, 0.641% Mn, 0.016% P, 0.131% S, 0.118% Cr, 0.02% Mo, 0.105% Ni and 0.451% Cu. The surfaces were polished with silicon carbide (particle size 25 μm), rinsed thoroughly with Milli-Q water and carefully dried. The working electrode was positioned horizontally with the

Linear voltammetry

First, the effects of the immersion time prior to the voltammetry experiments were investigated. The polarisation curves obtained after various immersion times in the solution containing 0.02 mol L−1 Na2HPO4 are presented in Fig. 1. In each case, the behaviour of carbon steel is typical of a passive material. For the smallest immersion times (15 min and 12 h), the metal is active above −600 mV/SCE, indicating that the pre-existing oxide film was reduced at least partially by the polarisation at

Discussion

Note first that a given experiment was repeated six times. We were particularly concerned with the reproducibility of our measurements as polarisation curves clearly revealed that the presence of phosphate species could induce important quantitative (but not qualitative) variations from one experiment to the other (see Fig. 2b). Of course, from a quantitative point of view (e.g. number of pits, maximum size of pits, maximum value reached by the current density, time at which the current density

Conclusion

Localised corrosion processes of carbon steel in NaHCO3 + NaCl stagnant aerated electrolytes were induced by anodic polarisation. The addition of Na2HPO4 solutions did not lead to the repassivation of steel inside well-grown pits, in contrast with what was previously observed by adding NaNO2 solutions in similar conditions [26]. The main beneficial effects of phosphates are more likely due to their adsorption on the passive iron oxide film that hinders the initiation of the pitting processes, and

Acknowledgement

The authors acknowledge the financial support by the Conseil General of Charente Maritime, France.

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